Amethyst is a purple-colored quartz, and was once considered to be in the same category as diamonds, sapphires, rubies, and emeralds. It has a chemical composition of SiO2, a trigonal crystal system, and (among other ways) it obtains its color from substitution by irradiation of trivalent iron Fe3+ for silicon. Amethyst is also the birthstone for February! Dna Necklace, Molecule Necklace, Crystals Amethyst, Science Jewelry, Tree Of Life Earrings, Lucky Gifts, Crystal System, Lapis Lazuli Necklace, Raw Amethyst